cxx: minor improvements

• Use ASIO's address_v4 instead of address class (we can't use IPv6 anyway and we can use our own tools to convert the IP to a string - this fixes a coverity defect since ASIO's to_string can throw for some reason)
• Use a global ZeroMQ context: this is nicer for debugging and also potentially beneficial since we only spawn one ZeroMQ thread, which we can more easily manipulate (e.g. setting number of IO threads, CPU affinity, etc)